1. Field of the Invention and Related Art Statement
The present invention relates to an optical current transformer for detecting an electric current conducting along a current conducting means arranged in electric power supply system or substation.
Hereinbefore, in the case of measuring an electric current conducting along a current conducting means of the power supply system or substation, it has been proposed to use a ceramic type current transformer. It should be noted that the ceramic type current transformer comprises an iron or magnetic core which is provided on the current conducting means to generate a magnetic flux proportional to the magnitude of the current and has an air gap, an opto-magnetic element arranged within the air gap of the iron core for sensing the magnetic flux, an insulator post for supporting the iron core, and an optical fiber having one end coupled with the opto-magnetic element and being inserted in a central hole formed in the insulator post. Light transmitted through the opto-magnetic element is detected by a photodetecting means to produce a signal which represents the magnitude of the current passing through the current conducting means. The above mentioned insulator post having the optical fiber inserted therethrough is known and is described in U.S. Pat. No. 4,810,836 issued on Mar. 7, 1898. In the known ceramic type current transformer, the current conducting along the current conducting means such as one or more wire conductors and conductive plates can be measured by detecting the light transmitted through the optical fiber. The known ceramic type current transformer is large in size, so that it is necessary to provide an additional space for arranging the ceramic type current transformer. In a recent power supply system for supplying a heavy current, a plurality of conductors are used to conduct a single phase current. For instance, two to eight conductors are grouped into a single current conducting means for conducting a single phase current. There have been proposed several methods for detecting the current conducting along such current conducting means. In one known method, a plurality of conductors belonging to the same current conducting means are bound into a single conductor and a large iron core is arranged around the conductor. In another known method, an iron core of the current transformer is arranged on one of a plurality of wire conductors of the single phase of the current conducting means. In the former method, since the current conducting means becomes very large, the iron core has to be made large and heavy, and thus the insulator post for supporting such a large and heavy iron core is liable to be large in size. Therefore, this known method could be applied only to such a place where a sufficiently large space for arranging the large insulator post is available. Moreover, in the first method, a plurality of conductors have to be cut and are then bound into the single conductor. Therefore, the operation for providing the current transformer requires long time and high cost. In the latter method, it is difficult to measure the current accurately. Further if any fault might occur on a wire conductor on which the current transformer is not arranged, the fault could not be detected any more.
In the known fault location detecting system for detecting a position at which a fault such as ground fault and short-circuit fault, directions of the fault current at respective detecting points are detected and then a fault location is determined by comparing the fault current directions at adjacent points. To this end, information representing the fault current direction detected at a fault detecting point has to be transmitted to an adjacent fault detecting point. Therefore, it is necessary to provide a rather complicated signal transmitting means between successive fault detecting points, resulting in a high fault location detecting system cost.